Flux for continuous casting of steel

ABSTRACT

A substantially entirely vitrified flux composition having flowidity and plastic deformation point suitable for use in the continuous casting of steel consists essentially of controlled proportions of alkali metal oxide, silica, calcium oxide, fluorine, and oxide of a Period IV metal having Atomic Number of 23-28, inclusive, desirably with phosphorus pentoxide and titanium dioxide. In a process for continuous casting of steel the exposed molten metal surface is covered with a layer of such composition.

United States Patent 1 1 Corbett [4 1 Aug. 12, 1975 1 1 FLUX FOR CONTINUOUS CASTING OF STEEL [75] Inventor: Paul M. Corbett, Baltimore, Md,

[73] Assignee: SCM Corporation, Cleveland, Ohio [22 Filed: Mar. 16, 1973 [21] Appl. No; 342,052

[52] US. Cl. 75/94; 75/96; 106/52; 148/26; 164/82 [51] Int. (31. C228 9/10; C03C 3/04; B221) 11/04 [58] Field of Search 75/94, 96; 117/6; 106/48, 106/52, 73.1, 51; 148/26; 164/82 [56] References Cited UNlTED STATES PATENTS 2,825,947 3/1958 Goss 164/73 X 3,318,363 5/1967 Goss 164/82 3,357,876 12/1967 Rinehart 106/52 X 3,433,611 3/1969 Saunders et 211.. 117/124 B X 3,454,433 7/1969 Mueller 148/22 3,642,052 2/1972 Schrewe et a1. 164/82 X 3,649,249 3/1972 Halley et a1 75/94 X 3,677,325 7/1972 164/82 X 3,698,466 10/1972 164/82 X 3,704,744 12/1972 Halley et a1 75/94 X Primary ExaminerAlle-n B. Curtis Assistant Examiner-Thomas A. Waltz Attorney, Agent, or Firm-Merton H. Douthitt; S. 1. Khayat [57] ABSTRACT 3 Claims, No Drawings FLUX FOR CONTINUOUS CASTING OF STEEL This invention is related to my copending US. Pat. application No. 290.196 filed Sept. lb. N72. and entitled Flux for Continuous Casting of Steel. That specification is expressly incorporated herein by reference. The present invention relates to improvements in flux compositions for continuous casting of steel. and to such process using such flux.

Various steels such as high quality deoxidized carbon steel and certain stainless steels can be and often are now made by the process of continuous casting. In such manufacture inclusions of various materials such as oxides. silicates. and aluminates are particularly undesirable because they can give rise to surface or subsurface defects. Additionally the mold desirably is protected at the top from attack by oxygen using a slag or flux. Such slag or flux can help to suppress the formation of these undesirable inclusions. as well as to lubricate the steel as it is going through the mold and to affect heat transfer from the hot steel to the cooledmold.

A desirable flux composition for such continuous casting must possess chemical and thermal stability during contact with the molten steel in order to avoid generation of appreciable quantities of objectionable fumes. toxic gases. decomposition products. or byproducts which might harm personnel or contaminate the product. Thus, lead-providing material in appreciable proportion is not used. Often such flux composition is called on to solubilize some of the impurities such as oxides. silicates. or aluminates which are believed to be the cause of most surface or subsurface imperfections found in continuous castings or in formed materials produced therefrom. The flux composition should have a softening point (or a plastic deformation point) and a flowidity such that the layer of flux on the molten steel maintains its effectiveness during the casting process. Various iluxing and refractory ingredients can be tolerated in small proportions in the resulting steel in some cases; in other instances they must be low or absent to suppress transfer to the steel. Hence. aluminumproviding material in a flux is undesirable for use with some aluminum-killed steels. but can be used with the casting of other steels; various Period lV metalproviding materials can be tolerated in fluxes for steels normally containing them; occasionally boron oxide is undesirable in fluxes for various steel.

Halley. in his US. Pat. No. 3.649.249. discloses a synthetic slag composition eminently suitable for use in the continuous casting of aluminum-killed steel. said slag having the composition: silica 10 to 55 percent. calcia to 40 percent. calcium fluoride to 40 percent. sodium oxide and potassium oxide 5 to 35%. lithium oxide and lithium fluoride 0.5 to percent. boria 0 to percent. with the provision that the boria. calcium fluoride and lithium fluoride represent in combination. more than 15 percent of the composition. all percentages being by weight. Halley also characterizes his slag composition as having several particular properties: specific flowidity and plastic deformation point. both of which can be determined by standarized methods shown in the patent. and solubility of alumina therein in excess of 20 percent.

Extensive testing of Halleys synthetic slag has shown it to be superior to any of the heterogeneous prior art slag (flux) compositions for suppressing the incidence of surface defects on such steels made by continuous casting. nevertheless. defects to the extent of about 10 percent often occur with such slag. Fortunately. such defects are not entirely of a serious type. However. even minor defects are undesirable because they require additional time. cost, and effort for correction.

Fluxes made in accordance with the present invention have been found quite efficient in the continuous casting of various steels. For eliminating defects they compare favorably with those heretofor produced. They can also be made to reduce attack on aluminagraphite molten metal inlet tubes (shrouds) to the continuous caster. Furthermore. the alumina content of the instant fluxes can be adjusted within the limits stated herein to further suppress alumina solubility from the metal at modest sacrifice in raising fusion temperature and lowering flowidity of the flux. The instant flux compositions do not appear to form undesirable immiscible liquid phases at the elevated continuous casting temperatures.

An important distinction here from my prior work. US. Pat. application No. 290.196. and from the Halley patent. is the incorporation of an oxide of a Period lV metal having Atomic Number of 23-28. inclusive. Such inclusion reduces the softening point of the flux and increases flowidity. thus giving the flux desirable flowidity even though it contains refractory ingredients in appreciable proportions. Another advantageous feature of the instant flux is the inclusion of a relatively small proportion of titanium dioxide. In small proportion (less than about 4 percent) such inclusion tends to increase flowidity and reduce softening temperature of the flux. but as larger proportions of titanium dioxide are added up to the limits stated. viscosity of the flux is increased and this tends to reduce flowidity for controlling the balance of flowidity with the other desirable properties.

The instant flux composition is substantially entirely in the vitreous state as frit particles. While it is possible to use very small proportions (30 percent or less of the frit) of finely ground additives with the frit. which additives can smelt down in the caster to augment the composition within the limits stated. the flux as all frit appears to operate the best and most reliably in the continuous casting of steel. The frit can be used as directly fritted or can be ground and classified according to desired size ranges. Usually it is cheaper to use the frit as it is fritted by quenching from a smelter operation. The flux composition in its broad and in its more advantageous oxide analyses consists essentially of:

in a process for the continuous casting of steel using an open-ended mold. the process is improved by covering the molten metal surface at the top of the mold with a layer of such flux composition (usually handled by scoop and maintained several inches thick).

When the foregoing compositional limitations are complied with. the flux flowidity will be in the usual range for continuous casting of steel tie. about 2-16 inches as measured according to U.S. Pat. No. 3,649,249) and advantageously about ()l(l inches. Similarly the Plastic Deformation Point of the flux will be between about l()()() and about l()()F.. useful for continuous casting of steel. The alumina solubility will be at most about -17 percent measured in accordance with the test shown in US. Pat No. 3.649.249. If the flux composition contains alumina. the alumina solubility will be diminished correspondingly The flux compositions can be smelted from actual oxides or preferably for economy from their conventional ceramic raw material equivalents. For example. some raw materials can be used to provide one or more ingredients of the flux such as sodium silicate which can provide sodium oxide as well as silicon dioxide. Similarly. the various carbonates are capable of providing the. requisite oxides such as alkali metal carbonates. Care should be taken not to include substantial amounts of hydrated components if fluorides are present in the composition because of the formation of volatile fluorides. It should be appreciated that high purity for the raw materials is not required. and the compositions in accordance with the present invention can have the ordinary small amounts of impurities encountered in ceramic practice without serious shortcomings.

For efficiency and economy the Period lV metal oxide is an iron oxide and for purposes of calculation in our batch this specification reckons the iron oxide as F0 0 although it can be charged to the batch in other form. Other Period lV metal oxides wherein the metal has Atomic Number of 23-28 include the oxides of cobalt. manganese. chromium. vanadium, and nickel. For the purposes of calculation herein cobalt oxide is reckoned as C0 0 manganese oxide as MnO chromium oxide as Cr- ,O vanadium oxide as V 0 and nickel oxide as NiO. Nickel oxide is not as potent a fluxing material as is iron oxide and is considerably more expensive. The oxides of manganese. chromium, and vanadium often are useful in specialty steels or in certain stainless steels. but ordinarily will be avoided in various high-quality. low-earbon steels. They have reasonable fluxing effectiveness. Cobalt oxide also has similar effectiveness for my use but is. of course. fairly expensive. These materials need not be charged as oxides to the batch but usually are. Zirconia is also a useful viscosity adjusting agent. amounts in excess of 1 percent tending to give increasing viscosity and lower flowidity. whereas very small fractions of l percent work oppositely.

Raw batch ingredients for the flux preferably are premixed in the dry state, then melted and cooled to form frit (i.e.. small vitreous particles). It should be noted that the fusion or smelting temperature for most compositions falling in the ranges specified herein will not exceed ()()F. The resulting frit usually is crushed and pulverized to form particles in fineness passing at least 20 mesh (Tyler Standard Sieve) and advantageously being mostly between and 100 mesh in size or even finer (e.g.. at least 50 percent passing lOO mesh). Al-

tcrnatively. often with advantage. the frit can be used directly from customary quenching. It has been found that the flux can be used in this particulate form in the continuous casting process by simply providing a layer on the surface of the molten metal at the top of the mold in the caster. An adequate layer of the flux usually is about 1 to 2 inches or more in thickness and is maintained in such thickness throughout the continuous casting process by periodic or continuous additions. Typically. the amount of the flux utilized is about 1 pound per ton and generally in the range of 2 to 1.5 pounds per ton of steel cast.

The improvement of the present invention will be more readily understood from the consideration of the following specific examples which are given for the purpose of illustration and are not intended to be limiting. All parts and percentages are by weight unless specified otherwise.

EXAMPLE I A flux composition was prepared by conventionally dry-mixing. fusing. and fritting conventional raw batch ingredients listed to yield frit of the following analysis:

The flowidity of the flux (according to the method discussed in US. Pat. No. 3,649,249. cited herein) was measured to be 4 inches. Similarly the Plastic Deformation Point was about 1300F. This flux was especially useful for covering the molten metal surface at the top of a continuous casting mold while preventing attack on alumina-graphite molten metal nozzles feeding said mold.

EXAMPLE 2 A flux composition was prepared by conventionally dry-mixing. fusing. and fritting conventional raw batch ingredients to yield frit of the following analysis:

Oxide Percent by Weight NtnO 22.2 Ll- 6.5 Ca() l4. P. ,o 17.0 Si(). 270 F 13.0 'BaO 4.2 Sr() 3.()

Raw Batch Composition Ingredient Percent by Weight -Continued (hide Percent by Weight Na:('( 7.8 l.i- 14.5 BaC();- 5M SrCQ, 4.1 Si(). 204

The flowidity of the flux (according to the method described in US. Pat. No. 3.649.249, cited herein) was 9 inches. Similarly the Plastic Deformation Point was about l()5()F. This flux composition was of a lower melting range and was especially useful as a starter flux in continuous casting of steel.

EXAMPLE 3 A flux composition was prepared by conventionally dry-mixing. fusing. and fritting conventional raw batch ingredients to yield frit of the following analysis:

The flowidity of this flux (according to the method disclosed in U.S. patent No. 3.649.249. cited herein) was measured to be 6 inches. Similarly the Plastic Deformation Point was about 1 15()F. The TiO was a valuable additive to reduce the softening range of the flux for use In continuous casting of steel. In other like fluxes the titanium dioxide content was varied from 1.2-5.6 parts but the 3 percent level of TiO gave the lowest softening range of such fluxes.

EXAMPLE 4 A flux C'OmPOSIIIOn was prepared by conventionally dry maxing. fusing. and fritting conventional raw batch ingredients to yield frit of the following analysis:

The flowidity of the flux (according to the method disclosed in US. Pat. No. 3,649.249. cited herein) was measured to be 5 inches. Similarly the Plastic Deformation Point was about 12()OF. The iron oxide was found to impart excellent flowidity in spite of the alumina content.

When iron oxide is added to the fluxes of Examples 1, 2. and 3. increased flowidity can be had; alternatively flowidity can be preserved even though refractory materials such as alumina and silica are raised somewhat in content.

EXAMPLE 5 A flux composition was prepared by conventionally dry-mixing. fusing. and fritting conventional raw batch ingredients to yield frit of the following analysis:

()xidc Percent by Weight Li O 3.0 NAnO 8.4 CaO 22.3 B\ O 4.0 2 13.4 SK): 28.8 F 15.1 e 0 5.2

Raw Butch Compositions Ingredient Percent by Weight C aF- 31.7 l la ,B.() 5.8 Na CQ 10.8 Spodumene 42.4 Feldspar 2.3 A1 0 1.8 F0 0;, .2

The flowidity of the flux (according to the method disclosed in US. Pat. No. 3.649249. cited herein) was measured to be 3 inches. Similarly the Plastic Deformation Point was about l5()()F. The combination of iron oxide and aluminum oxide was used to generate special hardness with reasonable flow. In this specification the flowidity tests were performed at 2600F.

What is claimed is:

1. In a process for the continuous casting of steel using an open-ended mold for the molten metal, the improvement which comprises covering the exposed surface of said molten metal with a layer of flux composition in the form of frit particles, said composition consisting of:

Oxide of Period IV metal having Atomic Number of Z3-2X. inclusive l1(1 Continued Ingredient Percent by Weight no -6 ZrO 0-5.

wherein said percentages are selected to total 100%. 2. The process of claim 1 wherein the oxide of Period IV metal is iron oxide.

3. A flux composition having flowidity and plastic deformation point suitable for use in the continuous casting of steel, said composition being substantially in the form of frit particles, and consisting of the following ingredientsz wherein said percentages are selected to total 

1. IN A PROCESS FOR THE CONTINUOUS CASTING OF STEEL USING AN OPEN-ENDED MOLD FOR THE MOLTEN METAL, THE IMPROVEMENT WHICH COMPRISES COVERING THE EXPOSED SURFACE OF SAID MOLTEN METAL WITH A LAYER OF FLUX COMPOSITION IN THE FORM OF FRIT PARTICLES, SAID COMPOSITION CONSISTING OF:
 2. The process of claim 1 wherein the oxide of Period IV metal is iron oxide.
 3. A flux composition having flowidity and plastic deformation point suitable for use in the continuous casting of steel, said composition being substantially in the form of Frit particles, and consisting of the following ingredients: 